Cassiopeia @ 15mm

[c3po]

hi all,

got back from the in-laws yesterday evening only to find beautiful clear skies right over my back garden (what a blessing!)... I immediately rushed indoors, grabbed my camera & tripod, and shot out into the garden for the next 30 minutes or so - then i got cold!

anyway, here's the result of a my attempt on Cassiopeia and the double cluster (NGC 884 & NGC 869) using my Fuji S7000 'bridge camera' and DSS to stack 10 subs + 5 darks all with the following settings:

• shutter: 15sec
  
• f-Stop: 3.0
  
• focal length: 15mm
  
• ISO: 400
  

Comments & Critiques welcomed as always....how do i improve otherwise?

[Demonperformer]

Nice widefield shot - just a pity you lost the fifth main star.

[c3po]

Nice widefield shot - just a pity you lost the fifth main star.

thanks! I think in my haste to set up before freezing the brass monkeys bits off (I was in jeans and t-shirt and the temps were speedily dropping) i set the camera up and although all 5 stars were visible in the viewfinder, i forgot that it shows 5% more area than the final image size - chopping the bottom (and that all important 5th star!) off in the process.

[jgs001]

Looks pretty good. Agreed about the 5th star ... At 15 seconds, it might be worth trying to capture some more frames... Perhaps try 30, or even 40... if you start to get too much smearing in the corners, then don't stack them all.

[c3po]

Thanks, valuable info I've no doubt. Will the extra frames really improve the image significantly, or just sharpen out the details?

[Photosbykev]

Most viewfinders show only 95% of the final image rather than more than the final image?

You will find that DSS crops the edges of the final image after stacking so you need to make an allowance for it.

30 second exposures and/or increasing the iso will help to capture more stars and image depth if the stars don't trail too much and the noise doesn't become an issue.

[jgs001]

more short exposures is not a substitute for more time per exposure, but it can help pull out faint detail that is already in there, as stacking more exposures improves the signal to noise ratio.

[c3po]

Most viewfinders show only 95% of the final image rather than more than the final image?

You will find that DSS crops the edges of the final image after stacking so you need to make an allowance for it.

that makes sense...so i can blame DSS for cropping too tightly?

In reply to:

30 second exposures and/or increasing the iso will help to capture more stars and image depth if the stars don't trail too much and the noise doesn't become an issue.

and

more short exposures is not a substitute for more time per exposure, but it can help pull out faint detail that is already in there, as stacking more exposures improves the signal to noise ratio.

Great advice once again, thanks to both of you.

So, if I'm understanding correctly, stacking more subs helps to increase the signal-to-noise ratio, and this in-turn aims to give clearer/more sharpened details, right? I'd assume that this is helped further using the camera's RAW mode rather than taking JPEG files. Unfortunately, I'm limited to 15s frames (even in 'full manual/bulb mode) and, so I'm told, the S7000's noise ration isn't as good as it could be but i'm sure these won't stop me getting better images in the future.

Also, one thing i neglected to ask previously is how wide should i zoom out? I understand that being zoomed out as far as possible helps to gather more light, but would zooming in, to say 30mm as opposed to 15mm in the above shot, help to bring out any further details or enhance the image at all?

[jgs001]

This may seem a little short, sorry on my phone. Stacking helps to lift faint stuff out of the background noise as long as there's something there in each sub. For instance I was able to get an image of the bubble nebula with my unmodded camera using 40 second exposures (I needed 170 of them to do it mind.) longer exposures could have captured more details.

As to zoom, it's not going to make a lot of difference although the longer the focal length the faster you will get star trails. Also often as you zoom in the lens stops down, meaning you let less light in and need longer exposures ... Which you can't get on a photo tripod.

Hth

[c3po]

This may seem a little short, sorry on my phone. Stacking helps to lift faint stuff out of the background noise as long as there's something there in each sub. For instance I was able to get an image of the bubble nebula with my unmodded camera using 40 second exposures (I needed 170 of them to do it mind.) longer exposures could have captured more details.

As to zoom, it's not going to make a lot of difference although the longer the focal length the faster you will get star trails. Also often as you zoom in the lens stops down, meaning you let less light in and need longer exposures ... Which you can't get on a photo tripod.

Hth

hi JGS, yes that helps loads, however short the reply, thanks!

i had noticed actually that zooming in reduced the f-stop. My maximum f-stop is f2.8, the above example was zoomed very slightly and had stopped down to f3.0, what would be a minimum f-stop using a 15s exposure, would f3.5 be pushing it or is this entirely camera/ccd dependent?

[jgs001]

that's getting into complicated territory... You want the lens to be as wide as possible, but still maintain good performance. That normally means stopping down a little, but I'm not sure whether with the smaller sensor you have, that's so much of an issue (edge performance is generally not so good). I'd start out, going as wide as possible and try at f/2.8 and see how you get on to be honest. It's much easier.

To make a constant aperture zoom is very complicated, which of course makes it cost a lot more money... if you look at Canon SLR zoom lenses for instance, you'll see that a constant aperture zoom, tends to be in the L (pro grade) lens line ups (excepting I think the EF-S17-55 f/2.8).

The f-stop, is actually the focal ratio of the lens (same as with a telescope), so focal length divided by aperture (which is why the number gets bigger as the hole closes down).

[gdheib0430]

Definitely @ 18mm go for a 30sec exposure minimum unless you can bump it up to say 40-45secs. @ 35mm I can manage 20s with no start trails and about 10s @ ~70mm. 18 looks like the way to go though longer exposure times and shorter f ratio should bring out a good bit of details.